Processes for the production of microporous films are well known in the art. For example, U.S. Pat. No. 3,870,593 (which is incorporated herein by reference) describes a process wherein a microporous film is produced by: (1) dispersing finely divided particles of a non-hygroscopic inorganic salt such as calcium carbonate in a polymer; (2) forming a film from the polymer; and (3) stretching the film to provide microporosity. Such microporous films are used for a variety purposes, such as breathable barriers (e.g., in diapers).
Although there are numerous prior art references which disclose microporous films, most (such as U.S. Pat. No. 4,353,945) do not define the stretching process other than to specify unidirectional or biaxial stretching. The three most common stretching techniques are MDO (machine direction orientation), tenter ovens, and intermeshing ring rolls (also called interdigitating rollers). MDO stretching units were available in the early days of microporous film from vendors such as Marshall and Williams, Inc. of Providence, R.I. Typical MDO stretching units have heated rolls and nips, with downstream rolls running at a faster speed in order to provide stretching in the machine direction only.
Tenter ovens were also available from several vendors, including Marshall and Williams. Tenter ovens function by grasping the edges of a film passing through a heated oven and stretching the film in a cross-machine direction. Films stretched in the cross-direction exit the oven substantially wider than their original width.
Intermeshing or interdigitating stretching devices were produced during this early period by vendors such as Biax-Fiberfilm of Neenah, Wis. U.S. Pat. No. 4,153,751, for example, describes the use of interdigitating rollers having grooves which extend substantially parallel to the axis of the rollers in order to stretch films in the cross-machine direction.
Methods of making composites of a microporous film and a nonwoven fabric are also known in the art. A microporous film may be bonded directly to the fabric by a variety of means, including adhesive, thermal, and/or ultrasonic bonding. As further discussed below, such composites have also been prepared by extrusion-coating a polymer extrudate onto a nonwoven fabric and then rendering the film microporous (such as by stretching).
It may also be desirable to stretch microporous film/fabric composites, however, stretching has its drawbacks. For instance, for microporous films, typical positive effects of stretching include higher vapor breathability and improved surface aesthetics. Vapor breathability (also referred to as water vapor transmission rate, xe2x80x9cWVTRxe2x80x9d) can be estimated by laboratory test methods, and Is a function of the size and frequency of the micropores in the film. Additional stretching of an already microporous film is known to increase the size of existing pores and create new pores. Therefore, highly stretched microporous films and microporous film/fabric composites generally have higher vapor breathability as compared to similar materials which have been stretched to a lesser degree.
Likewise, surface feel and drapability are known to be improved by stretching. Film/fabric composites tend to be more stiff and harsh than either of the individual components alone. Stretching such composites tends to break down the rigid structure, thereby providing a softer surface feel and improved drapability.
On the other hand, stretching microporous film/fabric composites can result in decreased bond strength and increased pinholing. Stretching improves the softness and drapability by destroying the connection between film and fabric. This results in decreased bond strength in the laminate. Stretching can also cause undesirable damage to the laminate, such as pinholing, tearing, or shredding of the film, the fabric, or the composite as a whole.
Rather than bonding a microporous film to a fabric, it is also possible to first bond a non-porous film to a fabric, and then stretch the resulting composite in order to render the film microporous. For example, U.S. Pat. No. 5,865,926 describes a method wherein the film/nonwoven composite is incrementally stretched. U.S. Pat. No. 5,910,225 (which is incorporated herein by reference) uses MDO stretching and/or tenter oven stretching. In some instances, the prior art methods have been only partially successful due to damage to the composite caused by the stretching process. Damage includes, but is not limited to, pinholes, tears, and other functional and aesthetic defects.
Similarly, U.S. Pat. No. 6,013,151 (which is incorporated herein by reference) teaches that a film/nonwoven fabric laminate can be made microporous and breathable upon incremental stretching at high speeds. The resulting microporous laminates have a high water vapor transmission rate (WVTR). It has also been found that a flat film/nonwoven laminate can be incrementally stretched more uniformly than an embossed film/nonwoven laminate. More uniform stretching provides higher WVTR and fewer pinholes.
The bonding of a film and fabric also may be carefully controlled to avoid creating other functional and aesthetic problems. For example, in the case of extrusion coating a polyethylene extrudate onto a spunbond polypropylene web, process conditions such as melt temperature and nip pressure determine the intrusion of the fibers into the film structure. At the minimum level of intrusion, however, the film and fabric have little or no bond, and therefore tend to delaminate. At the maximum level of intrusion, on the other hand, the film and fabric essentially mold together and become one. Such a laminate, however, acquires the worst properties of the two individual components and tends to be both rigid and fragile. Too much bond strength is also known to limit the amount of stretching which may be performed without the risk of forming pinholes. Simply stated, if the bond between film and fabric is too large, the stretched film will sometimes fracture prior to delaminating, leaving a pinhole.
There is a continuing need for improvements in the performance and appearance of microporous films and composites of microporous films and nonwoven fabrics. In particular, improvements are desired for producing microporous films and microporous film/fabric composites having higher breathability, while avoiding pinholes and other functional and aesthetic defects.
One embodiment of the present invention provides a method of making a microporous laminate sheet comprising a first film layer and a second layer. The method comprises:
(a) bonding a first film layer to a second layer in order to form a laminate sheet, wherein the first film layer includes a pore initiator; and
(b) stretching the laminate sheet using at least one CD intermeshing stretcher and at least one MDO stretching unit.
In one embodiment, the second layer comprises a fabric layer, whereas in another embodiment the second layer comprises another film layer which includes a pore initiator. In a particular embodiment of this method, the laminate sheet may be stretched by at least one CD intermeshing stretcher either immediately before or immediately after being stretched by at least one MDO stretching unit. The engagement depth of the CD intermeshing stretcher may be from about 0.025 to about 0.1 inches and the MDO stretch ratio may be between about 1.1:1 and about 4:1.
The film layer may be formed from a thermoplastic composition. When the second layer is a fabric, the step of bonding the film layer to the fabric layer may comprise extruding the thermoplastic composition onto said fabric layer. For example, the thermoplastic composition may be extruded into a cast roll nip station along with the fabric layer, wherein the cast roll nip station includes a pair of rollers having a nip therebetween.
The thermoplastic composition may be polyolefin based and comprise:
at least one polypropylene, polyethylene, or functionalized polyolefin; and
calcium carbonate as a pore iniator.
One particular composition comprises:
one or more polyethylenes;
about 40% to about 60% calcium carbonate; and
about 1% to 10% of one or more additives chosen from the group consisting of: pigments, processing aids, antioxidants, and polymeric modifiers.
The basis weight of the first film layer of the laminate may be between about 10 and about 40 gsm.
The fabric layer may be a polyolefin based nonwoven material. For example, the fabric layer may be chosen from the group consisting of: spunbond polypropylene; spunbond polyethylene; and carded, thermal bonded polypropylene. The basis weight of the fabric layer may be between about 10 and about 30 gsm, and the resulting laminate may have a water vapor transmission rate of greater than about 500 grams per square meter per day and a hydrohead in excess of about 60 cm.
Another embodiment of the present invention provides a method of making a microporous film, comprising the steps of:
(a) extruding a thermoplastic film from a polymer composition which includes a pore initiator; and
(b) stretching the film using at least one CD intermeshing stretcher and at least one MDO stretching unit.
In a particular embodiment, the microporous film is stretched by at least one CD intermeshing stretcher either immediately before or immediately after being stretched by at least one MDO stretching unit.
Yet another embodiment of the present invention provides a method of making a microporous laminate sheet comprising at least two film layers, comprising the steps of:
(a) bonding a first film layer to a second film layer in order to form a laminate sheet, wherein the first film layer includes a pore initiator; and
(b) stretching the laminate sheet using at least one CD intermeshing stretcher and at least one MDO stretching unit.
In a particular embodiment, each of the film layers is formed from a thermoplastic composition, and the step of bonding the first film layer to the second film layer comprises co-extruding said thermoplastic compositions.
The present invention also provides an apparatus for stretching a film or a film/fabric laminate, comprising a CD intermeshing stretcher and a MDO stretching unit, wherein the CD intermeshing stretcher and the MDO stretching unit are arranged such that a film or film/fabric laminate may be stretched by the CD intermeshing stretcher either immediately before or immediately after being stretched by the MDO stretching unit.